Originally Posted by PuttPutt

The drive side idler must endure significally higher loading than the return side idler. Higher loading means reduced idler and idler bearing life, elevaded noise and vibration, and requires a secure and robust mount. The return idler doesn't suffer the same loading, or the same frictional losses, or require heavy duty mounting - it is essentially, a low-loss chain guide. It's purely a guess on my part, but it looks like the LWB Bacchetta could do without the drive side idler. Unfortunately, if the drive side idler is removed, you would also eliminate the one-and-only place to mount the return idler.

By using a single member between the seat and the bottom bracket, the designer kills three birds with one stone; (1) Greater rigidity between the seat and the pedals, (2) any seat adjustments will retain identical rider-to-pedal relationship, and (3) is cheaper to mass produce. Huge advantages when selling a large number of bikes to a variety of leg lengths. Unfortunately, it doesn't leave the designer many elegant places to hang a return idler if one is needed.